1. Field of the Invention
The present invention concerns frames which mount an artist's canvas and more particularly frames which tension such canvas to eliminate any distortion or wrinkle lines developing on the surface of the canvas.
2. Description of the Relevant Art
An artist's canvas is held stretched taut upon a stretcher bar frame. The requirments of such a frame are that it should be optimally rigid against torsion and twisting, dimensionally stable during temperature or humidity changes, lightweight, and inexpensively constructed of readily available materials. Typically the frame is made of wood with mitered corners which may exhibit duck tail or other types of interlocking corner joints.
Over prolonged time periods distortion of a stretched-canvas frame is induced by longitudinal and lateral stresses arising from the canvas or from the environment. These stresses can cause deviation from the desired angle between frame members (normally a right angle) and/or a twisting or warping of the frame in its established plane. Such warpage or twisting of the frame typically causes stress to be developed in the canvas which, after prolonged duration, results in wrinkles being generated in the canvas. In an attempt to remedy such wrinkles, it has long been known to manually drive wooden wedges between adjacent ends of the frame members at the mitered corners of the frame, which cause a selective spreading of the frame members to re-stretch the canvas thereon. Such wooden wedges, however, typically weaken the corner joint integrity and often result in further frame warpage, thereby perpetuating the problem. In addition, the manual driving of the wedges into the corners of the frame often causes inadvertant cutting or tearing of the canvas due to abrasion of the wedge upon the rear surface of the canvas. Such inadvertant tearing, of course, results in permanent non-recoverable damage to the artist's rendition on the canvas. In view of recognition of this problem, certain framing solutions involving the long-term tensioning of canvas stretched upon a frame have been sought. Particularly, certain frames which constantly tension the canvas stretched upon such frames have been developed in the prior art.
One prior art frame of this type is the "GOLDLINE" constant tension, stretched-canvas frame available from H. W. Peel & Co., Ltd., Norwester House, Fairway Estate, Fairway Drive, Greenford, Middlesex, UB6 8PW, United Kingdom. This frame uses wood peripheral frame members which are mitered to approximately 45 degrees at their corners. An elongate metal spring clip is positioned between adjacent frame members at the junction of their mitered corners. This spring clip tends to force the adjacent frame members apart from each other. The frame members are not directly or rigidly affixed to each other. Rather, in order to hold each pair of adjacent frame members together in their desired spatial relationship defining a frame, a plastic corner piece is used at each corner joint. The plastic corner pieces slidably engage adjacent wooden frame members and lie over the top of the centrally positioned spring clip. The plastic corner pieces attempt to hold the wooden frame members, which are being forceably spread apart by the spring force generated in the intervening spring clip, in positional alignment even though the frame members are not in direct contact with one another. In maintaining this positional alignment, the plastic corner pieces attempt to suppress the torsional, longitudinal, and vertical movements which might otherwise result in a distortion of the frame. The plastic corner pieces further attempt to prevent twisting of adjacent frame members relative to one another, and to thereby prevent any resultant distortion of the plane of the canvas established by such frame. As will be discussed after further consideration of the construction of this prior art constantly tensioning, stretched-canvas frame, these attempts are not completely successful.
In order to assemble the "GOLDLINE" constant tension, stretched-canvas frame, the canvas is stretched over, and tacked to, the sides of the peripheral frame members while the frame members are held in alignment by the plastic corner pieces. The metal spring clips are then positioned between the mitered corners of adjacent frame members, thereby spreading the frame members apart and tensioning the canvas. In order that even further additional tensioning forces may be applied, it is also known in this prior art frame to position a central cross member consisting of two perpendicular arms within the rear interior region of the frame. Each arm of the cross member spans between an opposed pair of frame members. Each of four ends of the two cross member arms do not directly abut the frame member adjacent its end, but rather contact the frame member through additional intervening spring clips. Since the wooden pieces of the peripheral frame members and of the cross member are not directly connected, plastic guide pieces are used to maintain required alignments. The additional cross member, plastic corner pieces, and the plastic guide pieces undersirably add considerable weight and cost to the frame.
This particular "GOLDLINE" prior art stretcher frame is utilized to continuously present tensioning forces to the canvas which is stretched taut upon it. However, in order to obtain this tensioning, and the required "full-floating" relationship between all frame members, this prior art tensioning frame incurs a great penalty in the establishment and maintanence of a precision alignment between and among the frame members The plastic corner pieces must slidably engage the frame members over a large surface area, on the order of several square inches, in order to obtain an adequate grasp on such frame members for the purpose of establishing and maintaining their relative alignment. However, both the frame pieces and the plastic corner pieces are poorly adaptable to precision construction. Furthermore, the dimensions of the wooden frame members vary with temperature, humidity, and age relative to the retaining plastic corner pieces. These prior art corner plastic pieces, which slidably engage the frame members in order to guide them into alignment, have therefore proven to be generally inadequate for this task. Particularly, the "GOLDLINE" prior art stretcher frame does not exhibit an alignment between frame members which is as equivalently rigid, precise, or permanent to the alignment routinely attained by prior art fixed, non-tensioning frames with rigid corner joints.
Consequently, it is desired to produce a frame for the tensioning of stretched canvas which precisely establishes and permanently maintains a highly accurate angular positional relationship between frame members. From the perspective of the canvas stretched taut upon such a frame, the frame will expand in size as necessary to maintain a constant tension upon the canvas while resisting warpage or bow. Further, the shape of the frame will not change in symmetry from the desired polygonal shape in which the frame was initially constructed. For example, when such a frame is initially constructed as a rectangle then it will not, over time, assume the shape of a parallelogram wherein the corners of the frame are not at right angles. Neither will the frame members twist relative to one another. Finally, the frame should be of comparable wight and cost to conventional, non-tensioning frames.
There exists still another, more subtle, problem with prior art tensioning frames for canvas, including, with the "GOLDLINE", constant tensioning, stretch-canvas frame. Mainly, the prior art tension-providing spring clips or other steel spring members are often difficult to use safely in their operative position which is closely proximate to the artist's canvas. Any mispositioning, misinsertion, and/or misadjustment of the prior art spring clips may force them into contact with the canvas mounted upon the frame, and may even cause them to inadvertently tear the canvas in a manner analogous to the prior art use of wooden wedges.
Consequently, it is additionally desired that a constant-tensioning, stretched-canvas frame should be capable of being readily assembled, dissembled, and/or adjusted without hazard to the canvas mounted there upon. It is further desired that any permissible movement of the frame pieces over long periods of time should never hazard the canvas.
As a final, subsidiary, element of frame design it is known to chamfer the outer perimeter edges of peripheral frame members, which are nominally made of wood, in order to increase the surface area of contact of such frame members with the canvas stretched thereupon. However, the inner peripheral edges, and the inner peripheral surface adjacent to these edges, are normally at the same close spacing relative to the plane of the canvas as are the exterior circumferential edges and adjacent exterior peripheral surface of the same frame members. Consequently, if a canvas suffers inward pressure--such as may routinely be generated by thermal stress through the period of many years, the canvas is apt to pick up indentations, ultimately resulting in creases or wear areas at these pressure points or pressure lines whereat it contacts the inner periphery of the frame members. A canvas which suffers these indentations visually appears to assume a pressure imprint upon its front surface outlining its rearwardly-disposed frame members. This phenomenon of "frame marks", usually seen predominantly in older canvas, is highly undesirable. Consequently, in addition to all other requirements that the canvas should be securely, safely, and continuously tensioned and stretched in a flat plane, it is still further desired that the canvas should not be contacted by the frame during the long course of being maintained thereon at any location other than its periphery.
Although prior art approaches to the tensioning of canvas stretched taut upon a frame may be perceived to exist, in accordance with the preceding discussion it will be understood that the detailed requirements of optimally perfoming this task might be highly sophisticated. This is indeed the case, especially when it is considered that all applied solutions must perform satisfactorily over a period of time which often is conservatively measured in centuries.